Process for stabilizing a polyurethane



United States Patent PROCESS FOR STABILIZING A POLYURETHANE John S.Rngg, Wilmington, Del. assignor to E. I. du Pont de Nemonrs and Company,Wilmington, DeL, a corporaflon of Delaware No Drawing. Application April10 1953 Serial No. 348,113

2 Clnima. (Cl. 260-453) This invention relates to an improved processfor the manufacture of elastomers. The invention relates moreparticularly to the hot milling of stabilized condensat on polymers ofpolyalkylene ether glycols and arylene diisocyanates in which thepolymer chain has been extended by ction with water. The condensationpolymers with which this invention is concerned are prepared by thereaction of a polytetramethylene ether glycol with a tolylenediisocyanate and water, as more particularly described 111 co-pend1ngapplications Serial No. 297,990 filed July 9, 1952, by Hill, Serial No.288,531 filed May 17, 1952, by Langerak, Prucino and Remington, andSerial No. 28 8,532 filed May 17, 1952, by Langerak. The condensat onpolymer so formed is stabilized against premature curing by treatmentwith a nitrogen base containing at least one hydrogen attached to thenitrogen, as more particularly disclosed in co-pending application ofNelson and Verbanc, Serial No. 305,912 filed August 22, 1952. Apreferred stabilized condensation polymer may be prepared from 1 mol ofa polytetramethylene ether glycol of average molecular weight of 2000 to4000 reacted with from 2.0 to 2.5 mole of 2,4-tolylene diisocyanate, andthen w1th at least 0.5 mol of water, after which the polymer isstabilized by the addition of from 0.5 to 2.0 parts of piperidine perl00 parts of polymer. This product is an amber to light yellow, tough,rubbery polymer which can be milled to a smooth band on a rubber mill.

Condensation polymers prepared by the reaction of polyalkylene etherswith arylene dusocyanates and water can be cured by intimately mixingthe polymer with a diisocyanate and then heating in a mold underpressure. Once the diisocyanate is mixed with the condensation polymerthere is a tendency for reaction to take place which results in across-linking set-up that causes a premature cure or scorch of thepolymer. scorching may be defined as a decrease in thermoplast city dueto incipient vulcanization. scorching results 111 3. lack of buildingtack and interferes with or prevents typical rubber processingoperations such as milling, extrusion, calendering or molding, thusmaking fabrication into useful articles diflicult or impossible.

When the compounded polymer scorches at room tem; perature on standing,it is sometimes called bm scorch to difierentiate it from mill scorchwhich occurs on the rubber mill. The rate of scorching increases as thetemperature increases. Scorching on a mill frequently occurs in thecondensation polymers under consideration within from 1 to 5 minutesafter the diisocyanate is added, at temperatures of from 200 to 250 F.

Difierent lots of polyalkylene ether glycol-arylene diisocyanate-waterpolymers often exhibit different scorching rates or scorch times. Binscorch time is defined as the time interval between the addition of thediisocyanate curing agent and the time that the polymer begins to becomeunmillable. Mill scorch time is defined as the time interval between thestart of milling with additional diisocyanate and the time whenscorching becomes evident on the mill. The mill scorch time may varyfrom about 1 to about 20 minutes with various lots of polymer. It

2,702,797 Patented Feb. 22, 1955 I have found that the tetramethyleneether glycoltolylene diisocyanate condensation polymers which have beenextended by reaction with water can be improved materially in theirscorching properties by a somewhat prolonged milling at temperatures offrom about 300 to 360 F. During this milling the polymermay becomesufficiently tacky to require the use of a doctor blade to remove it.However, this processing may be carried out on conventional rubberindustry equipment which requires no particular modification.

The temperatures as above stated are relatively specific, for milling attemperatures substantially below 300 F. gives only slight improvement inthe scorch time of the polymers under consideration. If the temperatureis allowed to go much'above 360 F. the polymer has a tendency to becomefluid and it is ditficult to keep it on the mill. While milling at thetemperatures within the range above specified demonstrably affects thepolymer after about 10 minutes, it will be found that in most cases themilling should be carried out for at least 20 minutes and in some casesup to one hour. The time of milling will vary inversely with thetemperature employed since higher temperatures require shorter times. Ingeneral there is no added benefit in continuing the milling for morethan one hour, although longer time of milling does not appear to beharmful.

The following examples are given to illustrate the invention.

Example 1 A polymer is made by the following procedure: One molarproportion of polytetramethylene ether glycol (average molecular weight2900), which contains 0.24% of water-and has an acid number 1.6, is putin a Werner- Pfieiderer mill which is heated to about 75 C. to melt it.There is then added 2.3 molar proportions of 2,4-tolylene diisocyanateand milling is carried on for about 20 minutes at about 100 C. There isthen added 1.17 molar proportions of water and milling is continued for50 minutes. The mass then resembles crumbs. It is removed from theWerner-Pfieiderer mill and sheeted out to a smooth band on a rubbermill. There is then added immediately while working on the mill about 1part of piperidine per 100 The speed ratio of the front roll to the backroll is l.0:l.32 and the surface speed of the front roll is 38.3

- feet per minute. .The temperature of the rolls is conis sometimesimpossible to thoroughly mix the curing trolled at 3301-5 F. As soon asthe polymer has handed out, the working distance or nip is adjusted sothat an active rolling bank of stock about one-half inch in diameterforms at the nip. The polymer adheres to the mill and must be scrapedoff with a doctor knife. At the end of 20 minutes the polymer is removedfrom the mill.

parts of this milled polymer is then put on a cold rubber mill at about80 to 100 F. and 6 parts of the dimer of 2,4-tolylene diisocyanate aremilled in.

The scorch time is determined by putting a portion on an unheated rubbermill and milling and allowing the temperature to increase due tofriction and shearing without circulating any coolant through the mill.

A control sample which had not been hot milled is also compounded thesame way and tested for scorch. The samples are compared below:

Thusit will be seen that the control sample is completely scorchedbefore the hot milled sample shows the first evidence of scorch. Bycomplete scorch is meant the stage at which the polymer is so completelycured that it will no longer ride on the mill.

Example 2 Other portions of the polymer prepared in Example 1 aretreated the same way except that the curing agent is 2,4-tolylenediisocyanate. The improvement is shown In this case, the first evidenceof scorch in the milled polymer shows up more quickly than when thedimer is used but the complete scorch time is nearly double that of thecontrol.

. Example 3 The polymer is made as in Example 1 from:

1 mol polytetramethylene ether glycol (average molecular weight 3100,acid number 0.56, water 0.11%) 2.3 mols.2,4-tolylene diisocyanate 1.09mols water and is stabilized with 0.5 part of piperidine per 100 partsof polymer.

400 grams of the polymer are milled as in Example 1 at 300:5 F. for 40minutes. with 40 g. of the dimer of 2,4-tolylene diisocyanate and testedfor scorch time as before. The initial scorch time is greater than 7minutes while a control sample which has not been milled is scorchedbefore the dimer is completely dispersed in'the polymer. The milledsample is removed from the scorch test mill and cured in a mold underpressure for 60 minutes at 274 F. It has a tensile strength at the breakof 4575 pounds per square inch, and elongation at the break of 550%,amodulus at 300% elongation of 1175 pounds per square inch.

I Example 4 A polymer is made as in Example 1 from the following:

1.0 mol polytetramethylene ether glycol (average molecular weight 3350,acid number 0.82, water.-0.l8%)

It is then compounded mols 2,4-tolylene diisocyanate 1.09 mols water vand is stabilized with 1.39 parts of piperidine per 100 parts ofpolymer.

400 grams are then milled at 330;t5 F. on the 6 x 12 inch rubber millfor 10 minutes. It is removed and compounded with 40 grams of the dimerof 2,4-tolylene diisocyanate. A control sample is similarly compounded.Both samples are stored at room temperature for 2 days. On inspection atthe end of this time it is found that the control sample is severelyscorched, while the one milled at 330 F. is completely free of any signof vscorch.

Any of the stabilizing agents of the class consisting of nitrogen bases,havingat least one hydrogen attached to nitrogen, that are disclosed inco-pending application of Nelson and Verbanc Serial No. 305,912 may besubstituted for the piperidine used in the above examples. As theredisclosed, operable stabilizing agents of this class include ammonia andsalts such as ammonium carbonatehydrate which decompose. under reactionconditions to yield ammonia; alkyl amines, either saturated orunsaturated, such as monoor diethylamine, n-btuylamine, di-n-butylamine,diisobutylamine, and allylamine; aryl amines, such as aniline; aralkylamines, such as benzylamine; mixed alkyl-aryl secondary amines such asN-methyl aniline; alicyclic amines, such as cyclohexylamine anddicyclohexylamine; and heterocyclic nitrogen bases, such as piperidine,morpholine and ethyleneimme.

Any organic diisocyanate compound which under the curing conditions isconverted to a diisocyanate may be employed as a curing agent in thisprocess.

,I claim:

1. A process for improving the scorching properties of elastomerscomprising polytetramethylene ether glycoltoluene diisocyanate polymersin which the polymer chain has been extended by reaction with water andwhich have been stabilized b the addition of a nitrogen base having atleast one hy ogen attached to nitrogen, which comprises milling thestabilized polymer prior to the addition of a curing agent fort'rom- 10to minutes at temperatures of from about 300 F.'to about 360 F.

2. A process for improving the scorching properties of elastomerscomprising polytetramethylene ether glycoltoluene diisocyanate polymersin which the polymer chain has been extended by reactionwith water andwhich have been stabilized by the addition of piperidine, whichcomprises milling the stabilized polymer prior to the addition of acuring agent for from 10 to 60 minutes at temperatures of from about300' F. to about 360 F.

\ No references cited.

1. A PROCESS FOR IMPROVING THE SCORCHING PROPERTIES OF ELASTOMERSCOMPRISING POLYETRAMETHYLENE ETHER GYLCOLTOLUENE DIISOCYANATE POLYMERSIN WHICH THE POLYMER CHAIN HAS BEEN EXTENDED BY REACTION WITH WATER ANDWHICH HAVE BEEN STABILIZED BY THE ADDITION OF A NITROGEN BASE HAVING ATLEAST ONE HYDROGEN ATTACHED TO NITROGEN, WHICH COMPRISES MILLING THESTABILIZED POLYMER PRIOR TO THE ADDITION OF A CURING AGENT FOR FROM 10TO 60 MINUTES AT TEMPERATURES OF FROM ABOUT 300* F. TO ABOUT 360* F.